Beach Rambles in Search of Seaside Pebbles and Crystals / With Some Observations on the Origin of the Diamond and Other Precious Stones

by John George Francis

But let us now look into the back-parlour, where all the cutting and polishing takes place. A peep behind the scenes is generally instructive.

The first time I ever entered such a “sanctum,” I remained there nearly an hour; asking questions, you may be sure.

The lapidary works by means of wheels. These wheels are instruments for sawing and filing the stones which come under his hand: he has nothing to add to them, but a good deal to take away from them. When any one divides an orange with a sharp knife equatorially, he treats it as a choanite is treated in the simplest case: but the principle is somewhat different. The knife may cut the first as smooth as cheese, owing to the superior hardness of the steel and the compressibility of the fruit: but no instrument that man can invent will ever do this with a substance like flint or jasper. All that the wheel can do is to act like a saw, and to take out from the centre of the pebble, in the shape of saw-dust, a section equal in thickness to the wheel itself. If this be effected, and no more, then that part of the work is perfectly done: and this is not effected, without some fault, once in a dozen times, unless the workman be both skilful and attentive. The usual fault is, that the plane of section is not kept perfectly horizontal.

But let us now see how the wheels act. There must always be two of them: one which is to cut by its edge, and another which is to polish by its surface. But the lapidary is generally provided with several of both kinds; partly in case of accident, partly on account of variety in the texture of the pebbles brought to him. Each wheel is fixed upon an axis of its own, of perhaps a cubit’s length. And this portable axis is bevelled as a screw, for some inches, at the other end, so that it can be screwed at pleasure into a solid revolving spindle planted upright in the floor of the workshop. The spindle itself may either be worked by a leathern strap and a treddle; or, as is more usual, by a winch-handle acting at a mechanical advantage.

The horizontal wheel being made to spin round by turning the winch with one hand, the workman grasps the pebble firmly in the other, and presses it against the edge of the disk, which revolving in a horizontal plane acts like a saw. A pencil-line may first be drawn on the stone, to mark out the intended section.

But this is not all. A dry wheel, although it were formed of the finest steel, could barely scratch the surface of an agate: or, if great force were exerted at the winch, would splinter it. The lapidary has need of diamond-powder, emery, and rotten-stone. He makes use also of a peculiar kind of oil, and has a jar of water within reach for ordinary purposes. The oil is called “brick-oil.” It comes from coal-tar, and does not heat by friction, as common oil would: consequently, it neither burns the operator’s fingers, nor injures his specimens by causing the wheel to glow too fiercely. Of the above materials, the most expensive is the diamond. Although obtained by crushing “bort,” of little intrinsic value, it never costs the lapidary less than twenty-five shillings per carat; and he can do nothing without it. Emery, on the other hand, which is a coarse variety of corundum, is cheap enough.

The first wheel put on the spindle is an extremely thin one, it can scarcely be too thin, and is made of tin or of the softest steel. Its disk should lie exactly level in the plane of the horizon, and it must not have the shadow of a notch or bend upon its delicate edge. This wheel the lapidary wets, along its entire rim, with oil or water, by means of a feather. A small portion of the diamond, ready mixed with oil, is then applied to the edge of the wheel, the latter being made to revolve gently, until it has taken up the mixture from his finger.

The wheel thus primed is now set in motion, at first slowly, but by degrees more rapidly, and the pebble being steadily pressed, not pushed, against it, the diamond eats into the metal, and the metallic edge armed with an adamantine tooth eats into the solid stone, and at length saws it asunder.